(1) Technical Field
This invention generally relates to electronic circuits, and more specifically to radio frequency (RF) switch circuits utilizing field effect transistors (FETs).
(2) Background
Electronic signal switches are used in a wide variety of applications. In particular, a multiple-pole FET-based RF switch architecture has been found useful in applications which require multiple transmit and/or receive paths for RF signals. For example, such a switch architecture is useful in cellular radio systems for coupling multiple antennas to multiple sets of transmit/receive circuitry. Other examples include automatic test equipment and electronics laboratory bench measurement equipment.
A FET may be fabricated in various technologies (e.g., standard bulk silicon, silicon-on-insulator, silicon-on-sapphire, GaN HEMT, GaAs pHEMT, and MESFET processes) and is commonly represented in schematic diagrams as an idealized ON-OFF switch device. However, in many applications, particularly in RF circuits, the structure and materials of a FET may have significant effects on its own operation (e.g., with respect to bandwidth, isolation, and power handling) and the presence of a FET may have significant effects on other components in a circuit. Such effects arise in part because a “CLOSED” or “ON” (i.e., low impedance or conductive) FET has a non-zero resistance, and an “OPEN” or “OFF” (i.e., high impedance or blocking) FET behaves as a capacitor due to parasitic capacitances arising from the proximity of various semiconductor structures, particularly within the close confines of an integrated circuit (IC). Moreover, in a multiple-pole FET-based RF switch IC, added impedance elements (e.g., inductors and/or transmission lines) as well as parasitic inductances caused by interconnections between FETs and by other circuit structures complicate the design of such an IC.
To date, such complications have limited the number of available transmit and/or receive paths in a multiple-pole FET-based RF switch to about 8 ports (plus a common port), and RF performance has been less than stellar. In general, increasing the number of paths for an RF switch results in trade-offs with bandwidth and insertion loss (IL). Additionally, adding throw counts creates extra parasitic inductances along signal paths which degrade performance due to asymmetry when different signal paths are active. For example, a conventional 8 or 12 throw solid state switch has generally been limited to about a 3 GHz broadband response. As another example, some 8 throw switches fabricated in GaAs have been created with useful bandwidths up to about 8 GHz, but at the expense of insertion loss, return loss quality, isolation, and linearity/power compression point.
Accordingly, there is a need for a high throw-count multiple-pole FET-based RF switch architecture that provides good RF performance. The present invention meets such needs.